1. Field of the Invention
This invention relates to lubricating oil compositions and more particularly to a lubricating oil composition which is particularly suitable for lubricating machineries and implements for processing marine, farm, livestock and other food products.
2. Prior Art
Food processing for livestock, marine, farm or other stock materials involve the steps of selection, classification, pulverization, mixing, baking, heating, fermentation, boiling, drying, cooling and so on. These processing steps require their respective machines and instruments such as tea-leaf collecting machine, gain polishing machine, flour grinder, brewing machine, baking and confectioneries making machines, machines for making fruit juice, jams and pickles, milk processing machine, ham/sausage making machine, fish-meat and seaweed processing machine, vacuum film evaporator or kneader for preparing food additives, natural flavours and medicines, and many other machines and implements employed for making or processing a variety of food items.
Hithertofore used as lubricating oils for such machines and implements are a mineral oil or liquid paraffin, a liquid vegetable oil such as soybean oil, cottonseed oil and rape oil as well as an animal fat and oil such as beef or pork fat. Mineral oils and liquid paraffins are considered objectionable in terms of sanitation as they tend to move from the operative parts of the machine into and mingle with the food material being processed. While liquid vegetable fats and oils are hygienically acceptable, they are less oxidatively stable as may be determined by the Active Oxygen Method (AOM) hereinafter described.
A specific example of fats and oils (1) is disclosed in Japanese Laid-Open Patent Publication No. 56-72651 wherein a spray lubricant is proposed which comprises an ester exchange product of 30-90 parts by weight of an edible fat and oil containing 20 or less percent by weight of a saturated fatty acid and 70-10 parts by weight of a composition chiefly consisting of an ester of medium-chain saturated fatty acid and triglycerol hereinafter referred to as MCT having 6-10 carbon atoms.
Japanese Laid-Open Patent Publications 57-67695 and 62-32841 both propose the use of an ester-exchanged product of a fat and oil composition (2) derived from hydrogenative treatment of a vegetable oil such as camellia oil, tea blossom oil, olive oil, safflower oil, hazelnut oil and rape oil, the treated product having 16 or less percent by weight of linoleic acid and 12 or less percent by weight of saturated fatty acid.
Japanese Laid-Open Patent Publication 61-17343 discloses the use of a high-stability liquid oil (3) comprising an ester exchange product of a hydrogenated vegetable oil having 5 or less percent by weight of linoleic acid and MCT, which product has an AOM value of greater than 250 hours.
Japanese Laid-Open Patent Publication 62-3281 introduces a hydrogenated vegetable oil (4) of 80-95 iodine value ester-exchanged with a lauric acid, the ester-exchanged product having an AOM of 52-93 hours
Japanese Laid-Open Patent Publication 2-209995 discloses the use of a lubricating oil (5) for food processing machines which chiefly comprises triglyceride having a straight-chain alkyl group of 5-21 carbon atoms blended with a fatty acid of 12-22 carbon atoms.
The above lubricating oils (1)-(4) have a common drawback in that they are adequately resistant to oxidation due to the presence of increased unsaturated acids emanating from vegetable oils being hydrogenated to reduce polyenic acid contents and increase oleic acid contents.
The last-mentioned oil (5) is a MCT-based oil reputed for its high oxidative stability and low cloud point. This oil per se, however, has a relatively low viscosity ranging from 15-20 cp at 25.degree. C. to 10-15 cp at 40.degree. C. which is not readily adjustable to suit a particular application required by specific drive or operative component parts of a food processing machine.
There may be considered certain vegetable oils having a relatively high viscosity and such vegetable oils further hydrogenated or ester-exchanged. However, such vegetable oils are susceptible to deterioration by oxidation or solidification by polymerization due to the presence of unsaturated bonds in the fatty acid molecule, often resulting in seized machinery parts. It is known that linoleic acid and linolenic acid respectively have oxidation rates at 20.degree. C. of 12-20 times and about 25 times greater than that of oleic acid. Esters of saturated acid and glycerol are regarded to have a relatively high oxidative stability. Methyl stearate is known to have an oxidation rate about one-eleventh of that of methyl oleate or about one-hundredth of that of methyl linoleate.
Hydrogenation may be resorted to for reducing linoleic and linolenic acids in the glyceride with an increase in the contents of oleic acid to provide enhanced oxidative stability. However, this is not quite satisfactory because oleic acid is also an unsaturated fatty acid.
There may be considered certain highly viscous fats and oils such as those which contain large proportions of miristic acid, palmitic acid, stearic acid and other saturated fatty acids for use in the lubrication of food processing machineries. Such fats and oils are highly resistant to oxidation but have objectionably high cloud point, meaning high pour point and high freezing point. In order to reduce their cloud point, it may be possible to react high-viscosity fats and oils for ester exchange with MCT but only with appreciable effect.